[mythtv-users] Eliminating a ground loop/hum

[Robert Johnston]

On 7/5/06, Owen Mehegan <owen at nerdnetworks.org> wrote:
>
> I have a new Myth system that I've just built. I'm using an MSI PM8M-V
> motherboard, with its on-board audio (Via VT8233/A/8235/8237 AC97) for
> everything. The system is connected to a Sony 34" CRT TV using DVI/HDMI
> cable. Audio is routed as follows:
>
> cable box -> myth box -> tv -> Sony home theater receiver/speakers
>
> All of that is done via basic RCA cables. My problem is that I have an
> intermittent 60hz hum in the audio. I hear it whether I'm watching TV or
> just in the Myth interface. Playing with various mixer volumes doesn't make
> it go away, in the sense that I can't null it out without having the whole
> system's volume down too low to hear anything. I have everything plugged
> into one power strip/wall outlet, and the outlet has three prongs; whether
> it's really properly grounded or not is anyone's guess. I tried unplugging
> various unrelated components (Xbox, cable box, cable modem/router) and got
> no change.
>
> So, any suggestions on what I should do to clean up this audio? Maybe buy a
> better sound card? (And use spdif out while I'm at it? My receiver can
> handle it, though it's a pain in the ass from a "management of the available
> inputs" perspective.) What if I bought a UPS and plugged everything into
> that? I've seen suggestions of "home theater" magical grounding thingys...
> if you think one of those would help, a link to a specific product would be
> sweet. Thanks!

Using SPDIF would help a lot. But you could also try wiring the case
of the Amplifier to the case of the PC, which will establish a "Common
Ground" and prevent the hum (Or blow up the equipment because of the
voltage differential. Though that's not very likely as the act of
connecting the PC to the Amp via the audio cable would have made it
blow up too). Or you can cut the ground line in the RCA's to make a
"Ground Lift" and isolate the ground points of the two systems.

Personally, Though, I'd put everything through a single UPS, making
sure you use a nice smoothed output.
-- 
Robert "Anaerin" Johnston